The present invention relates to a video printer for making a hard copy of a video image, and more particularly to a video printer for making a hard copy of a single frame image as well as a composite image comprising a plurality of sub-frame images reduced in size and inlaid within a single frame.
Known video printers for making a hard copy of a video image include CRT type video printers, laser video printers, thermal transfer type video printers, ink jet type video printers and the like. Signals such as video signals from a video tape, real time video signals from a television camera, video signals from a video floppy on which images photographed by a electronic still camera have been recorded, are inputted to video printers of this type. An inputted video signal is supplied to a low-pass filter sampled and quantized by an A/D converter, and temporarily stored in a frame memory. The A/D converter samples the inputted video signal at a sampling frequency greater than twice the cut-off frequency of the low-pass filter so as to satisfy the sampling theorem. Video signals (image data) read out from the frame memory are subjected to color correction, size reduction/magnification, image synthesizing and the line and then are set to a printer unit to make a hard copy of the image data.
It has been desired for a video printer of this type to make an index sheet by which all scenes or main scenes can be seen at once. To this end, an attempt has been made to make a composite image of a plurality of sub-frame images reduced in size by inlaying the images within a single frame (such a composite image will be referred to hereinafter as a multifreeze image). An attempt also has been made to make a multi-print or a hard copy of the composite image, and to make a multi-print of a plurality of the same sub-frame images reduced in size and disposed in matrix fashion within the print, for the purpose of using each sub-frame image by attaching on a name card.
However, if a multifreeze image of a plurality of sub-frame images, e.g., 16 sub-frame images (being either the same or different), inlaid within a single frame is to be made, it becomes necessary to thin the video signal by "1/16". As a result, the resolution of the reproduced image is considerably degraded.
Thinning of the video signal is performed in order to make a multifreeze image of 4.times.4 sub-frame images so that the virtual sampling frequency at the A/D converter becomes "1/4". For example, even if a video signal of 4.5 MHz is sampled at 12 MHz. which is greater than 2.times.4.5 MHz, the virtual sampling frequency after the thinning becomes 3 MHz, thus failing to satisfy the sampling theorem. FIG. 4 illustrates such a case wherein a reproduced video signal (dotted line) is not a faithful reproduction of an original (inputted) video signal (solid line). A multi-print with poor reproduction fidelity will cause image quality deterioration such as moire with patterns resembling water ripples, or jagged slanted lines. Such image quality deterioration results from so-called turnaround noises due to lack of compliance with the Nyquist requirement (sampling theorem), as a result of which the lower side band component of the sampling frequency is superposed upon the original signal.